It has been a long-standing human necessity to seal access to certain assets, in particular assets in transit. In order to prevent smuggling, for instance, customs authorities routinely seal in-transit cargo vehicles and shipping containers with tagged sealing devices. Such tagged sealing devices may also be used by cargo owners or logistics providers to protect cargo against theft or other unauthorized tampering. While the physical protection offered by such tagged sealing devices may be limited, their main purpose is to clearly reveal whether they have been breached and the cargo potentially accessed. By regularly checking the tagged sealing devices, the cargo can be tracked and, if the sealing device has been breached, it is possible to identify the transit segment during which the breach has taken place. Such tagged sealing devices can thus be very efficient deterrents against unauthorized access, diversion and/or tampering of assets in transit.
Different types of tagged sealing devices are known to the skilled person. In one of its simplest forms, such a tagged sealing device may be a plastic tag with a ratchet strap. More elaborate tagged sealing devices take the form of a metal lock with a wire strap, a tagged metal strip seal, or a tagged bolt seal. To prevent false alerts, it is also important to prevent accidental breaches of such tagged sealing devices. To this purpose, the International Standard Organization (ISO) has issued the ISO 17712 standard for Tensile, Shear, Bend and Impact Resistance certification for sealing devices.
One inconvenience of most such tagged sealing devices is that checking and identifying them requires close visual inspection of the tags. When large numbers of containers have to be tracked, for instance in important customs checkpoints or transshipment facilities, this can be extremely tedious and time-consuming. Moreover, agents visually inspecting large numbers of seals may easily overlook individual seal breaches.
For this reason, several different types of secure sealing devices have been proposed incorporating radiofrequency identification (RFID) technology. Such RFID devices incorporate a radiofrequency transceiver for wirelessly communicating identification data to a remote reader. Moreover, the radiofrequency transceiver may also transmit a specific signal if the sealing device has been breached. For instance US Patent Application Publications US 2005/0231365 A1 and US 2007/0103310 A1 and U.S. Pat. No. 6,265,973 B1 each disclose a secure sealing device with an electronic circuit configured to transmit a specific signal in case of breach. However, the secure sealing devices disclosed in these documents require active radiofrequency transceivers, and thus a power supply. Ensuring such a power supply increases the complexity and cost of these sealing devices and reduces their reliability. The range at which such secure sealing devices can respond to a remote reader depends on this power supply. Increasing this range thus normally involves a trade-off in terms of increasing cost and decreasing reliability.
Alternatively, secure sealing devices have also been proposed that only require a passive radiofrequency transceiver, that is, a transceiver that can be powered solely by the energy of incoming radio signals. Such devices have been disclosed in US Patent Application Publications US 2006/0145868 A1, US 2006/0087431 A1 and US 2007/0139196 A1. In these secure sealing devices, an electrically conductive path connecting the radiofrequency transceiver and antenna is broken if the seal is breached. Such a breach can therefore be easily detected. However, one drawback of these secure sealing devices is that the range at which they respond to a remote reader is normally quite limited.